Radiation curable coating composition for optical fiber with reduced attenuation loss

ABSTRACT

The present invention relates to a radiation curable coating composition comprising a radiation curable oligomer comprising a backbone derived from polypropylene glycol and a dimer acid based polyester polyol, wherein said coating composition, when cured, is having: a) a hardening temperature (Th) of from −10° C. to about −20° C. and a modulus measured at said Th of lower than 5.0 MPa; or b) a hardening temperature (Th) of from −20° C. to about −30° C. and a modulus measured at said Th of lower than 20.0 MPa; or c) a hardening temperature (Th) of lower than about −30° C. and a modulus measured at said Th of lower than 70.0 MPa.

FIELD OF THE INVENTION

The present invention relates to a radiation curable coating compositionand to said radiation curable coating composition, which when disposedand cured as a first polymeric layer to surround an optical fiber, andwhen a second polymeric layer is disposed to surround said firstpolymeric layer results in an optical fiber having a reduced attenuationof the transmitted signal.

BACKGROUND ART

Optical fibers commonly consist of a glass portion (typically with adiameter of about 120-130 μm), inside which the transmitted opticalsignal is confined, The glass portion is typically protected by an outercoating, typically of polymeric material. This protective coatingtypically comprises a first coating layer positioned directly onto theglass surface, also known as the “primary coating”, and of at least asecond coating layer, also known as “secondary coating”, disposed tosurround said first coating. In the art, the combination of primarycoating and secondary coating is sometimes also identified as “primarycoating system”, as both these layer are generally applied during thedrawing manufacturing process of the fiber, in contrast with “secondarycoating layers” which may be applied subsequently. In this case, thecoating in contact with the glass portion of the fiber is called “innerprimary coating” while the coating on the outer surface of the fiber iscalled “outer primary coating”. In the present description and claims,the two coating layers will be identified as primary and secondarycoating, respectively, and the combination of the two as “coatingsystem”.

The thickness of the primary coating typically ranges from about 25 μmto about 35 μm, while the thickness of the secondary coating typicallyranges from about 10 μm to about 30 μm.

These polymer coatings may be obtained from compositions comprisingoligomers and monomers that are generally crosslinked by means of UVirradiation in the presence of a suitable photo-initiator. The twocoatings described above differ, inter alia, in the mechanicalproperties of the respective materials. As a matter of fact, whereas thematerial which forms the primary coating is a relatively soft material,with a relatively low modulus of elasticity at room temperature, thematerial which forms the secondary coating is relatively harder, havinghigher modulus of elasticity values at room temperature. The coatingsystem is selected to provide environmental protection to the glassfiber and resistance, inter alia, to the well-known phenomenon ofmicrobending, which can lead to attenuation of the signal transmissioncapability of the fiber and is therefore undesirable. In addition,coating system is designed to provide the desired resistance to physicalhandling forces, such as those encountered when the fiber is submittedto cabling operations.

The optical fiber thus composed usually has a total diameter of about250 μm. However, for particular applications, this total diameter mayalso be smaller; in this case, a coating of reduced thickness isgenerally applied.

In addition, as the operator must be able to identify different fiberswith certainty when a plurality of fibers are contained in the samehousing, it is convenient to color the various fibers with differentidentifying colors. Typically, an optical fiber is color-identified bysurrounding the secondary coating with a third colored polymer layer,commonly known as “ink”, having a thickness typically of between about 2μm and about 10 μm, or alternatively by introducing a colored pigmentdirectly into the composition of the secondary coating.

Among the parameters which characterize primary and secondary coatingsperformances, elastic modulus and glass transition temperature of thecross-linked materials are those which are generally used to define themechanical properties of the coating. When referring to the elasticmodulus it should be clarified that in the patent literature this issometimes referred to as “shear” modulus G (or modulus measured inshear), while in some other cases as “tensile” modulus E (or modulusmeasured in tension). The determination of said elastic moduli can bemade by means of DMA (Dynamic mechanical analysis) which is a thermalanalysis technique that measures the properties of the materials as theyare deformed under periodical stress. For polymeric materials, the ratiobetween the two moduli is generally 1:3, i.e. the tensile modulus of apolymeric material is typically about three times the shear modulus (seefor instance the reference book Mechanical Properties and Testing ofPolymers, pp. 183-186; Ed. G. M. Swallowe)

Examples of coating systems are disclosed, for instance, in U.S. Pat.No. 4,962,992. In said patent, it is stated that a soft primary coatingis more likely to resist to lateral loading and thus to microbending. Itthus teaches that an equilibrium shear modulus of about 70-200 psi(0.48-1.38 MPa) is acceptable, while it is preferred that such modulusbeing of 70-150 psi (0.48-1.03 MPa). These values correspond to atensile modulus of 1.4-4.13 MPa and 1.4-3.1 MPa, respectively. Asdisclosed in said patent, a too low equilibrium modulus may cause fiberbuckling inside the primary coating and delamination of the coatingsystem. In addition, said patents suggests that the glass transitiontemperature (Tg) of the primary coating material should not exceed −40°C., said Tg being defined as the temperature, determined by means ofstress/strain measurement, at which the modulus of the material changesfrom a relatively high value occurring in the lower temperature, glassystate of the material to a lower value occurring in the transitionregion to the higher temperature, elastomeric (or rubbery) state of thematerial.

Other examples of coating compositions are disclosed, for instance, inWO 01/05724, which discloses radiation curable fiber optic coatingmaterials comprising a (meth)acrylate urethane compound derived from apolypropylene glycol or comprising a (meth)acrylate urethane compoundderived from a polypropylene glycol and a further polyol including apolyester polyol. These compositions may be used, once cured, as coatingmaterial for optical fibers and optical fiber ribbons, including primarycoatings, secondary coatings, coloured secondary coatings, inks, matrixmaterials and bundling materials. In the introductory part, saiddocument mentions that primary coatings should in particular have a verylow Tg.

However, as noticed by the Applicant, although a primary coating has arelatively low value of Tg (as generally required by the art), the valueof the modulus of the coating material may nevertheless begin toincrease at temperatures much higher than the Tg, typically alreadyabove 0° C. Thus, while a low value of Tg simply implies that thetransition of said coating from its rubbery to its glassy state takesplace at relatively low temperatures, no information can be derived asto which would be the variation of the modulus upon temperaturedecrease. As a matter of fact, an excessive increase of the modulus ofthe primary coating may negatively affect the optical performances ofthe optical fiber, in particular at the low temperature values, thuscausing undesirable attenuation of the transmitted signal due tomicrobending.

Thus, as observed by the applicant, what seems important for controllingthe microbending of an optical fiber is the temperature at which thecoating material begins the transition from its rubbery state (soft) toits glassy state (hard), which temperature will be referred in thefollowing of this specification and claims as the “hardeningtemperature” of the material, or Th. In particular, attention should bepaid to select a composition which still shows a relatively low modulusat said Th, so that an excessive increase of the modulus upon furthertemperature decrease can be avoided.

In the present description and claims, the term “modulus” is referred tothe modulus of a polymeric material as determined by means of a DMA testin tension, as illustrated in detail in the test method section of theexperimental part of the present specification.

In the present description and claims, the term “hardening temperature”is referred to the transition temperature at which the material shows anappreciable increase of its modulus (upon temperature decrease), thusindicating the beginning of an appreciable change from a relatively softand flexible material (rubber-like material) into a relatively hard andbrittle material (glass-like material). The mathematical determinationof Th will be explained in detail in the following of the description.

According to the present invention, the Applicant has thus found thatattenuation losses caused by microbending onto a coated optical fibers,particularly at the low exercise temperatures, can be reduced bysuitably controlling the increase of the modulus at the lowtemperatures. In particular, the Applicant has found that saidmicrobending losses can be reduced by using a polymeric material for theprimary coating having a low hardening temperature and a comparativelylow modulus at said temperature. In addition, the Applicant has foundthat by selecting coating compositions having a relatively lowequilibrium modulus, said attenuation losses can be further controlledover the whole operating temperature range.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to aradiation curable coating composition wherein said composition comprisesa radiation curable oligomer comprising a backbone derived frompolypropylene glycol and a dimer acid based polyester polyol, andwherein said composition, when cured, is having:

-   -   a) a hardening temperature (Th) of from −10° C. to about −20° C.        and a modulus measured at said Th of less than 5.0 MPa; or    -   b) a Th of from −20° C. to about −30° C. and a modulus measured        at said Th of less than 20.0 MPa; or    -   c) a Th of less than about −30° C. and a modulus measured at        said Th of less than 70.0 MPa.

Preferably said composition forming said coating layer has:

-   -   a) a Th of from −10° C. to about −20° C. and a modulus measured        at said Th of less than 4.0 MPa; or    -   b) a Th of from −20° C. to about −30° C. and a modulus measured        at said Th of lower than 15.0 MPa; or    -   c) a Th of less than about −30° C. and a modulus measured at        said Th of lower than 50.0 MPa.

Preferably, the equilibrium modulus of said polymeric material is lowerthan about 1.5 MPa, more preferably lower than about 1.4 MPa, much morepreferably lower than about 1.3 MPa.

According to a preferred embodiment, the glass transition temperature ofthe material is not higher than about −30° C., more preferably nothigher than −40° C. and much more preferably not higher than −50° C.

Preferably, said composition, when disposed and cured as a firstpolymeric layer to surround a standard single mode optical fibercomprising an internal glass portion and when a second polymeric layeris disposed to surround said first polymeric layer, said optical fibershows an increase in the attenuation of the transmitted signal at 1550nm at a temperature of −30° C. of less than 1.5 (dB/km) (g/mm), morepreferably of less than 1.2 (dB/km) (g/mm), even more preferred lessthan 1.0 (dB/km) (g/mm), and most preferred, less than 0.8 (dB/km)(g/mm), when subjected to the expandable drum test.

Preferably, a standard single optical fiber according to the inventionshows a microbending sensitivity at 1550 nm at a temperature of −30° C.of less than 1.5 (dB/km)(g/mm) more preferably of less than 1.2(dB/km)(g/mm), even more preferred less than 1.0 (dB/km)(g/mm), and mostpreferred, less than 0.8 (dB/km)(g/mm), when subjected to the expandabledrum microbending test.

The term standard single mode fiber refers herein to optical fibershaving a refractive index profile of the step-index kind, i.e. a singlesegment profile, with a single variation of the refractive index of0.2%-0.4%, a core radius of about 4.0-4.5 μm and a MAC value of about7.8-8.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of an optical fiber according tothe invention;

FIG. 2 shows an illustrative DMA plot of a polymeric material accordingto the invention;

FIG. 3 shows the curve corresponding to the first derivative of the DMAplot of FIG. 2;

FIGS. 4 a to 4 c show the experimental DMA plots of three primarycoating materials suitable according to the invention;

FIG. 5 shows the experimental DMA plot of a prior art primary coatingmaterial.

FIG. 6 shows an illustrative embodiment of a drawing tower formanufacturing an optical fiber according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, an optical fiber according to the inventioncomprises an internal glass portion 101, a first polymeric coating layer102, also known as primary coating, disposed to surround said glassportion and a second polymeric coating layer 103, also known assecondary coating, disposed to surround said first polymeric layer.

As mentioned above, an optical fiber according to the present inventioncomprises a primary coating layer formed from a polymeric materialhaving a relatively low hardening temperature and a correspondingly lowmodulus at said temperature.

To better explain the meaning of the hardening temperature, reference ismade to the curve shown in FIG. 2. This curve, typically obtained by aDMA (Dynamic Mechanical Analysis), represents the variation of themodulus of a polymeric material vs. temperature. As shown by this curve,the polymeric material has a relatively high value of modulus at the lowtemperatures (glassy state, portion “a” of the curve), while said valuebecomes much lower when the polymer is in its rubbery state, at thehigher temperatures (portion “b” of the curve, equilibrium modulus). Theoblique portion “d” of the curve represents the transition of thematerial from the glassy to the rubbery state. The transition betweenthe glassy state and the rubbery state is known in the art as the “glasstransition” of the material and is generally associated to a specifictemperature (Tg, glass transition temperature). As apparent from thecurve, the transition between the glassy and the rubbery state takesplace over a relatively wide range of temperatures. For apparentpractical reasons, methods has thus been developed for determining aspecific Tg value for each polymer. One of this methods (see forinstance P. Haines, “Thermal Methods of Analysis”, p. 133. BlackieAcademic and professionals ed.), which is the one used for determiningthe Tg values indicated in the present description and claims, comprisesdetermining the intersection point of two lines. The first line(identified as “A” in FIG. 2) is determined by interpolating the pointsof the DMA curve in the plateau region of the glassy state (portion “a”of the curve). In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from −60° C. to−80° C. The second line (identified as “D” in FIG. 2) is determined asthe tangent to the inflection point of the DMA curve in the obliqueportion “d” of said curve. The inflection point and the inclination ofthe tangent in that point can be determined as usual by means of thefirst derivative of the DMA curve, as shown in FIG. 3. According to thecurve shown in FIG. 3, the abscissa of the minimum point of the curvegives the respective abscissa of the inflection point on the DMA curveof FIG. 2, while the ordinate gives the inclination (angularcoefficient) of the tangent line in said inflection point.

In practice, the derivative of each experimental point is firstcalculated and then the curve interpolating the derivative points isdetermined as known in the art. For avoiding unnecessary calculations,only those points falling within a relatively narrow temperature rangearound the minimum point are taken into account for the regression.Depending from the distribution of the experimental points, this rangemay vary between 40° C. (about ±20° C. around the minimum point) and 60°C. (about ±20° C. around the minimum point). A 6^(th) degree polynomialcurve is considered in general sufficient to obtain an curve to fit withthe derivative of the experimental points.

As shown in FIG. 2 the so determined glass transition temperature is ofabout −62° C.

Similarly to the Tg, also the hardening temperature (Th) of a polymericmaterial can be determined by the above method. The Th is thusdetermined as the intersection point between line “B” and the abovedefined line “D”, as shown in FIG. 2. Line “B” is determined byinterpolating the points of the DMA curve in the plateau region of therubbery state (portion “b” of the curve) i.e. at the equilibrium modulusof the material. In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from 20° C. and40° C.

As shown in FIG. 2, the Th calculated according to the above method willthus be of about −13° C.

As observed by the Applicant, when the cured material forming theprimary coating of the optical fiber has a Th lower than about −10° C.and a modulus lower than 5.0 MPa, preferably lower than about 4.0 MPa,at said temperature, the optical performance of the optical fiber can beimproved, particularly by reducing its microbending sensitivity,particularly at the low temperatures of exercise, e.g. below 0° C. As amatter of fact, the combination of these two parameters in a curedcomposition according to the invention applied as primary coating on anoptical fiber results in a relatively smooth increase of the modulusupon temperature decrease, thus allowing to control the microbendingphenomena down to the lower operating temperature limits, typically −30°C. As further observed by the Applicant, analogous control of themicrobending phenomena can be achieved also when the composition whencured, has a Th lower than −20° C. and a modulus at said temperaturelower than 20 MPa, preferably lower than 15 MPa, or when the curedcomposition has a Th lower than −30° C. and a modulus at saidtemperature lower than 70 MPa, preferably lower than 50 MPa.

The Applicant has further observed that if the equilibrium modulus ofsaid primary coating is lower than about 1.5 MPa, preferably lower thatabout 1.4 MPa, more preferably lower than 1.3 MPa, the microbendingsensitivity of the fiber can be further reduced, not only at the lowertemperatures of the operating range, but also at higher temperatures,e.g. at the room temperature. Said modulus should however preferably benot lower than about 0.5 MPa more preferably not lower than 0.8 MPa, inorder not to negatively affect other properties of the fiber.

Furthermore, the glass transition temperature of the composition of thepresent invention, after cure, which can be applied as primary coatingon an optical fiber is preferably not higher than about −30° C., morepreferably not higher than 40° C. and much more preferably not higherthan −50° C.

All the above indicated parameters, i.e. modulus, Th and Tg can bedetermined by subjecting a polymeric material to a DMA in tensionperformed according to the methodology illustrated in the experimentalpart of the present specification, and by evaluating the respective DMAplot of the material according to the above defined procedure.

Radiation-curable carrier systems which are suitable for forming acomposition to be used as primary coating in an optical fiber accordingto the invention contain one or more radiation-curable oligomers ormonomers (reactive diluents) having at least one functional groupcapable of polymerization when exposed to actinic radiation. Suitableradiation-curable oligomers or monomers are now well known and withinthe skill of the art. Commonly, the radiation-curable functionality usedis ethylenic unsaturation, which can be polymerized preferably throughradical polymerization. Preferably, at least about 80 mole %, morepreferably, at least about 90 mole %, and most preferably substantiallyall of the radiation-curable functional groups present in the oligomerare acrylate or methacrylate. For the sake of simplicity, the term“acrylate” as used throughout the present application covers bothacrylate and methacrylate functionality.

A radiation curable coating composition according to the presentinvention comprises a radiation curable oligomer, said oligomercomprising a backbone derived from polypropylene glycol and a dimer acidbased polyester polyol. Said radiation curable coating composition, whencured, may be used as a first polymeric layer to surround an opticalfiber comprising an internal glass portion, being denoted as a primarycoating for an optical fiber. Preferably, the oligomer is a urethaneacrylate oligomer comprising said backbone, more preferably a whollyaliphatic urethane acrylate oligomer.

The oligomer can be made according to methods that are well known in theart. Preferably, the urethane acrylate oligomer can be prepared byreacting

-   -   (A1) the polypropylene glycol, and    -   (A2) the dimer acid based polyester polyol,    -   (B) a polyisocyanate, and    -   (C) a (meth)acrylate containing a hydroxyl group.        Given as examples of the process for manufacturing the urethane        acrylate by reacting these compounds are    -   (i) reacting said glycol (A1 and A2), the polyisocyanate, and        the hydroxyl group-containing (meth)acrylate altogether; or    -   (ii) reacting said glycol and the polyisocyanate, and reacting        the resulting product with the hydroxyl group-containing        (meth)acrylate; or    -   (iii) reacting the polyisocyanate and the hydroxyl        group-containing (meth)acrylate, and reacting the resulting        product with said glycol; or    -   (iv) reacting the polyisocyanate and the hydroxyl        group-containing (meth)acrylate, reacting the resulting product        with said glycol, and reacting the hydroxyl group-containing        (meth)acrylate once more.

Polypropylene glycol (A1)—as used herein—is understood to refer to apolypropylene glycol comprising composition having a plurality ofpolypropylene glycol moieties. Preferably, said polypropylene glycol hason average a number average molecular weight ranging from 1,000 to13,000, more preferably ranging from 1,500 to 8,000, even more preferredfrom 2,000 to 6,000, and most preferred from 2,500 to 4,500. Accordingto a preferred embodiment, the amount of unsaturation (referred to themeq/g unsaturation for the total composition) of said polypropyleneglycol is less than 0.01 meq/g, more preferably between 0.0001 and 0.009meq/g.

Polypropylene glycol includes 1,2-polypropylene glycol,1,3-polypropylene glycol and mixtures thereof, with 1,2-polypropyleneglycol being preferred. Suitable polypropylene glycols are commerciallyavailable under the trade names of, for example, Voranol P1010, P 2001and P 3000 (supplied by Dow), Lupranol 1000 and 1100 (supplied byElastogran), ACCLAIM 2200, 3201, 4200, 6300, 8200, and Desmophen 1111BD, 1112 BD, 2061 BD, 2062 BD (all manufactured by Bayer), and the like.Such urethane compounds may be formed by any reaction technique suitablefor such purpose.

Dimer acid based polyester polyol (A2)—as used herein—is understood torefer to a hydroxyl-terminated polyester polyol which has been made bypolymerizing an acid-component and a hydroxyl-component and which hasdimer acid residues in its structure, wherein said dimer acid residuesare residues derived from the use of a dimer acid as at least part ofthe acid-component and/or by the use of the diol derivative of a dimeracid as at least part of the hydroxyl-component.

Dimer acids (and esters thereof) are a well known commercially availableclass of dicarboxylic acids (or esters). They are normally prepared bydimerizing unsaturated long chain aliphatic monocarboxylic acids,usually of 13 to 22 carbon atoms, or their esters (e.g. alkyl esters).The dimerization is thought by those in the art to proceed by possiblemechanisms which include Diels-Alder, free radical, and carbonium ionmechanisms. The dimer acid material will usually contain 26 to 44 carbonatoms. Particularly, examples include dimer acids (or esters) derivedfrom C-18 and C-22 unsaturated monocarboxylic acids (or esters) whichwill yield, respectively, C-36 and C-44 dimer acids (or esters). Dimeracids derived from C-1 8 unsaturated acids, which include acids such aslinoleic and linolenic are particularly well known (yielding C-36 dimeracids).

The dimer acid products will normally also contain a proportion oftrimer acids (e.g. C-54 acids when using C-18 starting acids), possiblyeven higher oligomers and also small amounts of the monomer acids.Several different grades of dimer acids are available from commercialsources and these differ from each other primarily in the amount ofmonobasic and trimer acid fractions and the degree of unsaturation.

Usually the dimer acid (or ester) products as initially formed areunsaturated which could possibly be detrimental to their oxidativestability by providing sites for crosslinking or degradation, and soresulting in changes in the physical properties of the coating filmswith time. It is therefore preferable (although not essential) to usedimer acid products which have been hydrogenated to remove a substantialproportion of the unreacted double bonds.

Herein the term “dimer acid” is used to collectively convey both thediacid material itself or ester-forming derivatives thereof (such aslower alkyl esters) which would act as an acid component in polyestersynthesis and includes (if present) any trimer or monomer.

The dimer acid based polyester polyol preferably has on average a numberaverage molecular weight ranging from 1,000 to 13,000, more preferablyranging from 1,500 to 8,000, even more preferred from 2,000 to 6,000,and most preferred from 2,500 to 4,000.

Examples of these dimer acid based polyester polyols are given in EP 0539 030 B1 which polyols are incorporated herein by reference. Ascommercially available products, Priplast 3190, 3191, 3192, 3195, 3196,3197, 3198, 1838, 2033 (manufactured by Uniqema), and the like can begiven.

The ratio of polypropylene glycol to dimer acid based polyester polyolin the oligomer may be ranging from 1:5 to 5: 1, preferably ranging from1:4 to 4:1, and more preferably ranging from 1:2 to 2:1, even morepreferably, polypropylene glycol and dimer acid based polyester polyolare present in an equimolar ratio.

Given as examples of the polyisocyanate (B) are 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 1,3-xylylene diisocyanate,1,4-xylylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylenediisocyanate, p-phenylene diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethanediisocyanate, 3,3′-dimethylphenylene diisocyanate, 4,4′-biphenylenediisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate,methylenebis(4-cyclohexylisocyanate), 2,2,4-trimethylhexamethylenediisocyanate, bis(2-isocyanatethyl)fumarate, 6-isopropyl-1,3-phenyldiisocyanate, 4-diphenylpropane diisocyanate, hydrogenateddiphenylmethane diisocyanate, hydrogenated xylylene diisocyanate,tetramethyl xylylene diisocyanate, lysine isocyanate, and the like.These polyisocyanate compounds may be used either individually or incombinations of two or more. Preferred isocyanates are tolylenedi-isocyanate, isophorone di-isocyanate, and methylene-bis(4-cyclohexylisocyanate). Most preferred are wholly aliphatic basedpolyisocyanate compounds, such as isophorone di-isocyanate, andmethylene-bis (4-cyclohexylisocyanate).

Examples of the hydroxyl group-containing acrylate (C) include,(meth)acrylates derived from (meth)acrylic acid and epoxy and(meth)acrylates comprising alkylene oxides, more in particular,2-hydroxyethyl(meth)acrylate, 2-hydroxypropylacrylate and2-hydroxy-3-oxyphenyl(meth)acrylate. Acrylate functional groups arepreferred over methacrylates.

The ratio of the polyol (A) [said polyol (A) comprising (A1) and (A2)],the polyisocyanate (B), and the hydroxyl group-containing acrylate (C)used for preparing the urethane acrylate is determined so that 1.1 to 3equivalents of an isocyanate group included in the polyisocyanate and0.1 to 1.5 equivalents of a hydroxyl group included in the hydroxylgroup-containing (meth)acrylate are used for one equivalent of thehydroxyl group included in the polyol.

The number average molecular weight of the urethane (meth)acrylateoligomer used in the composition of the present invention is preferablyin the range from 1200 to 20,000, and more preferably from 2,200 to10,000. If the number average molecular weight of the urethane(meth)acrylate is less than 100, the resin composition tends tosolidify; on the other hand, if the number average molecular weight islarger than 20,000, the viscosity of the composition becomes high,making handling of the composition difficult.

The urethane (meth)acrylate oligomer is preferably used in an amountfrom 10 to 90 wt %, more preferably from 20 to 80 wt %, even morepreferably from 30 to 70 wt. %, and most preferred from 40 to 70 wt. %of the total amount of the resin composition. When the composition isused as a coating material for optical fibers, the range from 20 to 80wt. % is particularly preferable to ensure excellent coatability, aswell as superior flexibility and long-term reliability of the curedcoating.

A radiation-curable composition according to the invention may alsocontain one or more reactive diluents (B) that are used to adjust theviscosity. The reactive diluent can be a low viscosity monomer having atleast one functional group capable of polymerization when exposed toactinic radiation. This functional group may be of the same nature asthat used in the radiation-curable oligomer. Preferably, the functionalgroup of each reactive diluent is capable of copolymerizing with theradiation-curable functional group present on the otherradiation-curable diluents or oligomer. The reactive diluents used canbe mono- and/or multifunctional, preferably (meth)acrylate functional.

A suitable radiation-curable primary coating composition comprises fromabout 1 to about 80 wt. % of at least one radiation-curable diluent.Preferred amounts of the radiation-curable diluent include from about 10to about 60 wt. %, more preferably from about 20 to about 55 wt. %, evenmore preferred ranging from 25 to 40 wt. %, based on the total weight ofthe coating composition.

Generally, each reactive diluent has a molecular weight of less thanabout 550 and a viscosity of less than about 500 mPa.s For example, thereactive diluent can be a monomer or a mixture of monomers having anacrylate or vinyl ether functionality and a C₄-C₂₀ alkyl or polyethermoiety. Examples of acrylate functional monofunctional diluents areacrylates containing an alicyclic structure such as isobornyl acrylate,bornyl acrylate, dicyclopentanyl acrylate, cyclohexyl acrylate, and thelike, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxybutylacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, isopropylacrylate, butyl acrylate, amyl acrylate, isobutyl acrylate, t-butylacrylate, pentyl acrylate, isoamyl acrylate, hexyl acrylate, heptylacrylate, octyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate,nonyl acrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate,dodecyl acrylate, lauryl acrylate, stearyl acrylate, isostearylacrylate, tetrahydrofurfuryl acrylate, butoxyethyl acrylate,ethoxydiethylene glycol acrylate, benzylacrylate, phenoxyethylacrylate,polyethylene glycol monoacrylate, polypropylene glycol monoacrylate,methoxyethylene glycol acrylate, ethoxyethyl acrylate,methoxypolyethylene glycol acrylate, methoxypropylene glycol acrylate,dimethylaminoethyl acrylate, diethylaminoethyl acrylate,7-amino-3,7-dimethyloctyl acrylate, acrylate monomers shown by thefollowing formula (1),

wherein R⁷ is a hydrogen atom or a methyl group, R⁸ is an alkylene grouphaving 2-6, and preferably 2-4 carbon atoms, R⁹is a hydrogen atom or anorganic group containing 1-12 carbon atoms or an aromatic ring, and r isan integer from 0 to 12, and preferably from 1 to 8.

Of these, in order to obtain a cured polymeric material having asuitably low hardening temperature and a suitably low modulus at saidtemperature, long aliphatic chain-substituted monoacrylates, such as,for example decyl acrylate, isodecyl acrylate, tridecyl acrylate, laurylacrylate, and the like, are preferred and alkoxylated alkyl phenolacrylates, such as ethoxylated and propoxylated nonyl phenol acrylateare further preferred.

Examples of non-acrylate functional monomer diluents areN-vinylpyrrolidone, N-vinyl caprolactam, vinylimidazole, vinylpyridine,and the like.These N-vinyl monomers preferably are present in amountsbetween about 1 and about 20% by weight, more preferably less than about10% by weight, even more preferred ranging from 2 to 7% by weight.

According to a preferred embodiment, the radiation curable compositionaccording to the invention comprises at least one monofunctionalreactive diluent (having an acrylate or vinyl ether functionality), saidmonofunctional diluent(s) being present in amounts ranging from 10 to 50wt. %, preferably ranging from 20 to 40 wt. %, more preferably from 25to 38 wt. %. The amount of mono-acrylate functional reactive diluentspreferably ranges from 10 to 40 wt. %, more preferably from 15 to 35 wt.% and most preferred from 20 to 30 wt. %.

The reactive diluent can also comprise a diluent having two or morefunctional groups capable of polymerization. Examples of such monomersinclude: C₂-C₁₈ hydrocarbondiol diacrylates, C₄-C₁₈ hydrocarbondivinylethers, C₃-C₁₈ hydrocarbon triacrylates, and the polyetheranalogues thereof, and the like, such as 1,6-hexanedioldiacrylate,trimethylolpropane triacrylate, hexanediol divinylether,triethyleneglycol diacrylate, pentaerythritol triacrylate, ethoxylatedbisphenol-A diacrylate, and tripropyleneglycol diacrylate.

Such multifunctional reactive diluents are preferably (meth)acrylatefunctional, preferably difunctional (component (B1)) and trifunctional(component (B2)).

Preferably, alkoxylated aliphatic polyacrylates are used, such asethoxylated hexanedioldiacrylate, propoxylated glyceryl triacrylate orpropoxylated trimethylol propane triacrylate.

Preferred examples of diacrylates are alkoxylated aliphatic glycoldiacrylate, more preferably, propoxylated aliphatic glycol diacrylate. Apreferred example of a triacrylate is trimethylol propane triacrylate.

According to a preferred embodiment the radiation curable compositionaccording to the invention which can be used as a primary caoting on anoptical fiber comprises, a multifunctional reactive diluent in amountsranging from 0.5-10 wt. %, more preferably ranging from 1 to 5 wt. %,and most preferred from 1.5 to 3 wt. %.

Without being bound to any particular theory, the present inventorsbelieve that the combination of the oligomer according to the presentinvention in amounts of less than about 75 wt. % (preferably less thanabout 70 wt. %) with a total amount of monofunctional reactive diluentsof at least about 15 wt. % (more preferably, at least about 20 wt. %,even more preferably at least about 25 wt. % and most preferred at leastabout 30 wt. %) aids in achieving a primary coating composition, thatafter cure, has an acceptably low hardening temperature and low modulusat said temperature.

It is further preferred that the composition comprises a mixture of atleast two monofunctional reactive diluents, more preferably, one of saidreactive diluents being substituted with a long aliphatic chain; evenmore preferably, the composition contains two long aliphaticchain-substituted monoacrylates. Preferably, at least about 10 wt. %,more preferably at least about 12 wt. % is present of said at least onelong aliphatic chain-substituted monoacrylate.

A liquid curable coating composition according to the present inventionsuitable to be applied as a primary coating layer on an optical fibercan be cured by radiation. Here, radiation includes infrared radiation,visible rays, ultraviolet radiation, X-rays, electron beams, α-rays,β-rays, γ-rays, and the like. Visible and UV radiation are preferred.

The liquid curable resin composition according to the present inventionpreferably comprises a photo-polymerization initiator. In addition, aphotosensitizer can be added as required. Given as examples of thephoto-polymerization initiator are 1-hydroxycyclohexylphenyl ketone,2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde,fluorene, anthraquinone, triphenylamine, carbazole,3-methylacetophenone, 4-chlorobenzophenone, 4,4′-dimethoxybenzophenone,4,4′-diaminobenzophenone, Michler's ketone, benzoin propyl ether,benzoin ethyl ether, benzyl methyl ketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanethone,diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1 -one,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,bis-(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and the like.

Examples of commercially available products of the photo-polymerizationinitiator include IRGACURE 184, 369, 651, 500, 907, 1700, 1750, 1850,819, Darocur 11 16, 1173 (manufactured by Ciba Specialty Chemicals Co.,Ltd.), Lucirin LR8728 (manufactured by BASF), Ebecryl P36 (manufacturedby UCB), and the like.

The amount of the polymerization initiator used can range from 0.1 to 10wt %, and preferably from 0.5 to 7 wt %, of the total amount of thecomponents for the resin composition.

In addition to the above-described components, various additives such asantioxidants, UV absorbers, light stabilizers, silane coupling agents,coating surface improvers, heat polymerization inhibitors, levelingagents, surfactants, colorants, preservatives, plasticizers, lubricants,solvents, fillers, aging preventives, and wettability improvers can beused in the liquid curable resin composition of the present invention,as required. Examples of antioxidants include Irganox1010, 1035, 1076,1222 (manufactured by Ciba Specialty Chemicals Co., Ltd.), Antigene P,3C, FR, Sumilizer GA-80 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of UV absorbers include Tinuvin P,234, 320, 326, 327, 328, 329, 213 (manufactured by Ciba SpecialtyChemicals Co., Ltd.), Seesorb 102, 103, 110, 501, 202, 712, 704(manufactured by Sypro Chemical Co., Ltd.), and the like; examples oflight stabilizers include Tinuvin 292, 144, 622LD (manufactured by CibaSpecialty Chemicals Co., Ltd.), Sanol LS770 (manufactured by Sankyo Co.,Ltd.), Sumisorb TM-061 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of silane coupling agents includeaminopropyltriethoxysilane, mercaptopropyltrimethoxy-silane, andmethacryloxypropyltrimethoxysilane, and commercially available productssuch as SH6062, SH6030 (manufactured by Toray-Dow Corning Silicone Co.,Ltd.), and KBE903, KBE603, KBE403 (manufactured by Shin-Etsu ChemicalCo., Ltd.). The viscosity of the liquid curable coating compositionaccording to the present invention which can be applied as a primarycoating layer on an optical fiber is usually in the range from 200 to20,000 cP, and preferably from 2,000 to 15,000 cP.

The primary coating compositions according to the present invention,when cured, typically have an elongation-at-break of greater than 80%,more preferably of at least 110%, more preferably at least 150% but nottypically higher than 400%.

The compositions according to the present invention will preferably havea cure speed of 1.0 J/cm² (at 95% of maximum attainable modulus) orless, more preferably about 0.7 J/cm² or less, and more preferably,about 0.5 J/cm² or less, and most preferred, about 0.4 J/cm² or less.

A radiation curable coating composition according to the presentinvention is preferably disposed and cured as a first polymeric layer tosurround an optical fiber (referred to as “primary coating”) and asecond polymeric layer is preferably disposed and cured to surround saidfirst polymeric layer (referred to as “secondary layer”). Preferably,said secondary coating is also based on a radiation curable composition.

The aforedescribed primary coating is then in turn coated with asecondary coating, of a type known in the art, compatible with theprimary coating formulation. For example, if the primary coating has anacrylic base, the secondary coating will also preferably have an acrylicbase.

Typically, an acrylic based secondary coating comprises at least oneoligomer with acrylate or methacrylate terminal groups, at least oneacrylic diluent monomer and at least one photoinitiator.

The oligomer represents generally 40-80% of the formulation by weight.The oligomer commonly consists of a polyurethaneacrylate.

The polyurethaneacrylate is prepared by reaction between a polyolstructure, a polyisocyanate and a monomer carrying the acrylic function.

The molecular weight of the polyol structure is indicatively between 500and 6000 u.a.; it can be entirely of hydrocarbon, polyether, polyester,polysiloxane or fluorinated type, or be a combination thereof. Thehydrocarbon and polyether structure and their combinations arepreferred. A structure representative of a polyether polyol can be, forexample, polytetramethylene oxide, polymethyltetramethylene oxide,polymethylene oxide, polypropylene oxide, polybutylene oxide, theirisomers and their mixtures. Structures representative of a hydrocarbonpolyol are polybutadiene or polyisobutylene, completely or partlyhydrogenated and functionalized with hydroxyl groups.

The polyisocyanate can be of aromatic or aliphatic type, such as thosepolyisocyanates illustrated previously. The monomer carrying the acrylicfunction comprises groups able to react with the isocyanic group; suchas the hydroxyl group-containing acrylates as illustrated previously.

The epoxyacrylate is prepared by reacting the acrylic acid with aglycidylether of an alcohol, typically bisphenol A or bisphenol F.

The diluent monomer represents 20-50% of the formulation by weight, itsmain purpose being to cause the formulation to attain a viscosity ofabout 5 Pas at the secondary coating application temperature. Thediluent monomer, carrying the reactive function, preferably of acrylictype, has a structure compatible with that of the oligomer. The acrylicfunction is preferred. The diluent monomer can contain an alkylstructure, such as isobornylacrylate, hexanediacrylate,dicyclopentadiene-acrylate, trimethylolpropane-triacrylate, or aromaticsuch as nonylphenyletheracrylate,polyethyleneglycol-phenyletheracry-late and acrylic derivatives ofbisphenol A.

Examples of photoinitiator(s) and further additive(s) that may be usedin the secondary coating composition are as illustrated previously.

A typical formulation of a cross-linkable system for secondary coatingscomprises about 40-70% of polyurethaneacrylate, epoxyacrylate or theirmixtures, about 30-50% of diluent monomer, about 1-5% of photoinitiatorand about 0.5-5% of other additives.

An example of a formulation usable as the secondary coating of theinvention is that marketed under the name of DeSolite® 3471-2-136 (DSM).The fibres obtained thereby can be used either as such within opticalcables, or can be combined, for example in ribbon form, by incorporationinto a common polymer coating, of a type known in the art (such asCablelite® 3287-9-53, DSM), to be then used to form an optical cable.

Typically, the polymeric material forming the secondary coating has amodulus E′ at 25° C. of from about 1000 MPa to about 2000 MPa and aglass transition temperature (measured as above defined) higher thanabout 30° C., preferably higher than 40° C. and more preferably higherthan about 50° C.

An optical fiber comprising a primary coating comprising a curedcomposition according to the present invention may be produced accordingto the usual drawing techniques, using, for example, a system such asthe one schematically illustrated in FIG. 6.

This system, commonly known as “drawing tower”, typically comprises afurnace (302) inside which a glass optical preform to be drawn isplaced. The bottom part of the said preform is heated to the softeningpoint and drawn into an optical fiber (301). The fiber is then cooled,preferably to a temperature of at least 60° C., preferably in a suitablecooling tube (303) of the type described, for example, in patentapplication WO 99/26891, and passed through a diameter measurementdevice (304). This device is connected by means of a microprocessor(313) to a pulley (310) which regulates the spinning speed; in the eventof any variation in the diameter of the fiber, the microprocessor (313)acts to regulate the rotational speed of the pulley (310), so as to keepthe diameter of the optical fiber constant. Then, the fiber passesthrough a primary coating applicator (305), containing the coatingcomposition in liquid form, and is covered with this composition to athickness of about 25 μm-35 μm. The coated fiber is then passed througha UV oven (or a series of ovens) (306) in which the primary coating iscured. The fiber coated with the cured primary coating is then passedthrough a second applicator (307), in which it is coated with thesecondary coating and then cured in the relative UV oven (or series ofovens) (308). Alternatively, the application of the secondary coatingmay be carried out directly on the primary coating before the latter hasbeen cured, according to the “wet-on-wet” technique. In this case, asingle applicator is used, which allows the sequential application ofthe two coating layers, for example, of the type described in patentU.S. Pat. No. 4,474,830. The fiber thus covered is then cured using oneor more UV ovens similar to those used to cure the individual coatings.

Subsequent to the coating and to the curing of this coating, the fibermay optionally be caused to pass through a device capable of giving apredetermined torsion to this fiber, for example of the type describedin international patent application WO 99/67180, for the purpose ofreducing the PMD (“Polarization Mode Dispersion”) value of this fiber.The pulley (310) placed downstream of the devices illustrated previouslycontrols the spinning speed of the fiber. After this drawing pulley, thefiber passes through a device (311) capable of controlling the tensionof the fiber, of the type described, for example, in patent applicationEP 1112 979, and is finally collected on a reel (312).

An optical fiber thus produced may be used in the production of opticalcables. The fiber may be used either as such or in the form of ribbonscomprising several fibers combined together by means of a commoncoating.

EXAMPLES

The present invention will be explained in more detail below by way ofexamples, which are not intended to be limiting of the presentinvention.

Coating Compositions

Coating compositions have been prepared to be applied as primary coatingon optical fibers. The compositions to be applied as a primary coatingon an optical fiber according to the invention are indicated as theexamples Ex.1, Ex.2 and Ex.3 in the following Table 1. TABLE 1 Radiationcurable primary coating compositions Ex. 1 Ex.2 Ex.3 (Wt. %) (Wt. %)(Wt. %) Oligomer I 68.30 60.30 67.30 Ethoxylated nonyl 10.00 19.00 10.00phenol acrylate Tridecyl acrylate 10.00 10.00 10.00 Long aliphaticchain- 2.00 2.00 2.00 substituted monoacrylate Vinyl caprolactam 5.006.00 5.00 Ethoxylated bisphenol 1.00 — 3.00 A diacrylate Trimethylolpropane 1.00 — — triacrylate (TMPTA) 2,4,6-trimethylbenzoyl 1.40 1.401.40 diphenyl phosphine oxide Thiodiethylene bis [3-(3,5- 0.30 0.30 0.30di-tert-butyl-4- hydroxyphenyl) propionate]) hydrocinnamate γ-mercaptopropyl 1.00 1.00 1.00 trimethoxysilane

Oligomer I is the reaction product of isophorone diisocyanate (IPDI),2-hydroxyethylacrylate (HEA), polypropylene glycol (PPG) and a dimeracid based polyester polyol.

In addition, comparative commercial primary coating DeSolite® 3471-1-129(as Comparative Experiment, Comp. Exp. A in table 2) has also beentested.

The equilibrium modulus, the Tg, the Th and the modulus at the Th foreach of the above cured primary coating compositions were as given inTable 2 (see test method section for details on DMA test anddetermination of respective parameters on the DMA curve). Thecorresponding DMA curves of said cured coating compositions are reportedin FIGS. 4A to 4C, respectively. TABLE 2 Parameters of cured primarycoating compositions Tg Th E′ E′(Th) Ex. 1 −59.1 −12.2 1.1 3.5 Ex. 2−56.6 −10.8 0.7 2.0 Ex. 3 −63.2 −13.3 1.1 2.7 Comp. Exp. A −55.1 −5.61.9 3.6Preparation of Optical Fibers

Coated standard single mode optical fibers have been manufactured asindicated in the test method section, by using the primary coatingcompositions of Examples 1-3 (fibers F-1, F-1 a, F-2 and F-3) or ofComparative Experiment A (fiber F-C) as the primary coating, togetherwith the commercial secondary coating DeSolite® 3471-2-136.

The single mode optical fibers that have been manufactured are given inTable 3 below. TABLE 3 Single mode optical fibers Primary Fiber coatingMAC F-1 Ex. 1 8.0 F-1a Ex. 1 7.9 F-2 Ex. 2 7.9 F-3 Ex. 3 8.35 F-C Comp.Exp. A 8.23

The MAC value for each fiber is determined as indicated in the testmethod section.

Microbending Tests

The results of the microbending test (see details in the test methodssection) on single mode optical fibers are reported in the followingtable 4. TABLE 4 Microbending on SM fibers Microbending Sensitivity(dB/Km)/(g/mm) Fiber MAC −30° C. +22° C. +60° C. F-1 8.00 0.75 0.4 1.6F1a 7.91 0.45 0.31 1.5 F-2 7.9 0.4 0.2 1.3 F-3 8.35 0.5 0.3 1.6 F-C 8.231.6 1.4 2.6

As shown by the above results, an optical fiber comprising a curedcoating composition according to the invention is less prone toattenuation losses caused by the microbending phenomenon, both at thelow as well as high operating temperatures.

Test Methods and Methods of Manufacturing

Curing of the Primary Coatings for Mechanical Testing (SamplePreparation)

A drawdown of the material to be tested was made on a glass plate andcured using a UV processor in inert atmosphere (with a UV dose of 1J/cm², Fusion D-lamp measured with EIT Uvicure or International Light IL390 B Radiometer). The cured film was conditioned at 23±2° C. and 50±5%RH for a minimum of 16 hours prior to testing.

A minimum of 6 test specimens having a width of 12.7 mm and a length of12.5 cm were cut from the cured film.

Dynamic Mechanical Testing

The DMTA testing has been carried out in tension according to thefollowing methodology.

Test samples of the cured coating compositions of examples 1-3 and ofcomparative experiment A were measured using a Rheometrics SolidsAnalyzer (RSA-11), equipped with:

-   -   1) a personal computer having a Windows operating system and        having RSI Orchestrator® software (Version V.6.4.1) loaded, and    -   2) a liquid nitrogen controller system for low-temperature        operation.

The test samples were prepared by casting a film of the material, havinga thickness in the range of 0.02 mm to 0.4 mm, on a glass plate. Thesample film was cured using a UV processor. A specimen approximately 35mm (1.4 inches) long and approximately 12 mm wide was cut from adefect-free region of the cured film. For soft films, which tend to havesticky surfaces, a cotton-tipped applicator was used to coat the cutspecimen with talc powder.

The film thickness of the specimen was measured at five or morelocations along the length. The average film thickness was calculated to±0.001 mm. The thickness cannot vary by more than 0.01 mm over thislength. Another specimen was taken if this condition was not met. Thewidth of the specimen was measured at two or more locations and theaverage value calculated to ±0.1 mm.

The geometry of the sample was entered into the instrument. The lengthfield was set at a value of 23.2 mm and the measured values of width andthickness of the sample specimen were entered into the appropriatefields.

Before conducting the temperature sweep, moisture was removed from thetest samples by subjecting the test samples to a temperature of 80° C.in a nitrogen atmosphere for 5 minutes. The temperature sweep usedincluded cooling the test samples to about −60° C. or about −90° C. andincreasing the temperature at about 2° C./minute until the temperaturereached about 100° C. to about 120° C. The test frequency used was 1.0radian/second. In a DMTA measurement, which is a dynamic measurement,the following moduli are measured: the storage modulus (also referred toas elastic modulus) E′, and the loss modulus (also referred to as theviscous modulus) E″. The lowest value of the storage modulus E′ in theDMTA curve in the temperature range between 10 and 100° C. measured at afrequency of 1.0 radian/second under the conditions as described indetail above is taken as the equilibrium modulus of the coating.

The corresponding DMA curves are reported in FIGS. 4 a to 4 c (examples1-3 respectively) and FIG. 5 (comp. Exp. A).

Determination of Glass Transition Temperature (Tg) and HardeningTemperature (Th)

Based on the respective DMA plot of each cured primary coating material,the Tg, Th and modulus at Th of the material have been determined asmentioned in the descriptive part.

Thus, with ref. to FIG. 2, the Tg is determined by the intersectionpoint of line A with line D. Line A is determined by interpolating thepoints of the DMA curve in the plateau region of the glassy statein thefollowing manner. First of all, the median value of logE′ in the regionfrom −60° C. to −80° C. is calculated. Line A is then determined as thehorizontal line (parallel to the x axis) passing through said value ofLogE′. Line D is determined as the tangent to the inflection point ofthe DMA curve in the oblique portion “d” of said curve. The inflectionpoint and the inclination of the tangent in that point are determined bymeans of the first derivative of the DMA curve; the abscissa of theminimum point of the derivative curve gives the respective abscissa ofthe inflection point on the DMA curve, while the ordinate gives theinclination (angular coefficient) of the tangent line in said inflectionpoint. The derivative curve has been determined by calculating thederivative of each experimental point of the DMA curve and then fittingthese points by means of a 6^(th) degree polynomial curve in the range+20/−40° C. around the minimum calculated derivative points.

Similarly, also the Th has been determined as the intersection point ofline B with line D (see FIG. 2). Line D is as above determined, whileline B is determined by interpolating the points of the DMA curve in theplateau region of the rubbery state in the following manner. First ofall, the median value of logE′ in the region from 20° C. to 40° C. iscalculated. Line B is then determined as the horizontal line (parallelto the x axis) passing through said median value of LogE′.

Manufacturing of Optical Fibers

All the optical fibers used in the present experimental section havebeen manufactured according to standard drawing techniques, by applyinga first (primary) coating composition on the drawn optical fiber, curingsaid coating composition and subsequently applying the secondary coatinglayer and curing it. The fiber is drawn at a speed of about 20 m/s andthe cure degree of the coating layers is of at least 90%. The curedegree is determined by means of MICRO-FTIR technique, by determiningthe amount of the reacted acrylate unsaturations (in terms of % RAU) inthe final cross-linked resin with respect to the initial photo-curablecomposition, e.g. as described in WO 98/50317.

Microbending Tests

Microbending effects on optical fibers were determined by the“expandable drum method” as described, for example, in G. Grasso and F.Meli “Microbending losses of cabled single-mode fibers”, ECOC '88, pp.526-ff or as defined by IEC standard 62221 (optical fibers measurementmethods—microbending sensitivity—method A, expandable drum publishedOctober 2002). The test is performed by winding a 100 m length fiberwith a tension of 55 g on a 300 mm diameter expandable metallic bobbin,coated with rough material (3M Imperials PSA-grade 40 μm).

The bobbin is connected with a personal computer which controls:

-   -   the expansion of the bobbin (in terms of variation of fiber        length); and    -   the fiber transmission loss.

The bobbin is then gradually expanded while monitoring fibertransmission loss versus fiber strain.

The pressure exerted onto the fiber is calculated from the fiberelongation by the following formula: $p = \frac{{EA}\quad ɛ}{R}$where E is the elastic modulus of glass, A the area of the coated fiberand R the bobbin radius.

For each optical fiber, the MAC has been determined as follows:${MAC} = \frac{MFD}{\lambda_{co}}$where MFD (mode field diameter according Petermann definition) at 1550nm is determined according to standard ITUT G650 and λ_(co) (lambdafiber cutoff—2 m lenght) is determined according to standard ITUT G650.

1. Radiation curable coating composition comprising a radiation curableoligomer comprising a backbone derived from polypropylene glycol and adimer acid based polyester polyol, wherein said coating composition,when cured, is having: a) a hardening temperature (Th) of from −10° C.to about −20° C. and a modulus measured at said Th of lower than 5.0MPa; or b) a hardening temperature (Th) of from −20° C. to about −30° C.and a modulus measured at said Th of lower than 20.0 MPa; or c) ahardening temperature (Th) of lower than about −30° and a modulusmeasured at said Th of lower than 70.0 MPa.
 2. Coating compositionaccording to claim 1, said composition having: a) a hardeningtemperature (Th) of from −10° C. to about −20° C. and a modulus measuredat said Th of lower than 4.0 MPa; or b) a hardening temperature (Th) offrom −20° C. to about −30° C. and a modulus measured at said Th of lowerthan 15.0 MPa; or c) a hardening temperature (Th) of lower than about−30° C. and a modulus measured at said Th of lower than 50.0 MPa. 3.Coating composition according to claim 1, wherein the equilibriummodulus of said composition, when cured, is lower than about 1.5 MPa. 4.Coating composition according to claim 1, wherein the equilibriummodulus of said composition, when cured, is lower than about 1.4 MPa. 5.Coating composition according to claim 1, wherein the equilibriummodulus of said composition, when cured, is lower than about 1.3 MPa. 6.Coating composition according to claim 1, wherein the glass transitiontemperature of said composition, when cured, is not higher than about−30° C.
 7. Coating composition according to claim 1, wherein the glasstransition temperature of said composition, when cured, is not higherthan about −40° C.
 8. Coating composition according to claim 1, whereinthe glass transition temperature of said composition, when cured, is nothigher than about −50° C.
 9. Cured coating for an optical fiber primarycoating comprising a radiation curable coating composition according toclaim 1, after cure.
 10. Coating composition system, said systemcomprising a primary coating composition and a secondary coatingcomposition for use as an optical fiber dual coating system, wherein theprimary coating composition is a composition according to claim
 1. 11.(canceled)
 12. (canceled)
 13. (canceled)
 14. Method for formulating aradiation curable coating composition comprising: mixing together atleast a radiation curable oligomer comprising a backbone derived frompolypropylene glycol and a dimer acid based polyester polyol, a reactivediluent and a photoinitiator, such that said composition, when cured, ishaving: a) a hardening temperature (Th) of from −10C to about −20° C.and a modulus measured at said Th of lower than 5.0 MPa; or b) ahardening temperature (Th) of from −20° C. to about −30° C. and amodulus measured at said Th of lower than 20.0 MPa; or c) a hardeningtemperature (Th) of lower than about −30° C. and a modulus measured atsaid Th of lower than 70.0 MPa.
 15. Method for making a cured primarycoating for an optical fiber comprising: i) formulating a radiationcurable coating composition comprising: a radiation curable oligomercomprising a backbone derived from polypropylene glycol and a dimer acidbased polyester polyol, a reactive diluent and a photoinitiator, and ii)curing said composition with ultraviolet light, such that saidcomposition, when cured, is having: a) a hardening temperature (Th) offrom −10° C. to about −20° C. and a modulus measured at said Th of lowerthan 5.0 MPa; or b) a hardening temperature (Th) of from −20° C. toabout −30° C. and a modulus measured at said Th of lower than 20.0 MPa;or c) a hardening temperature (Th) of lower than about −30° C. and amodulus measured at said Th of lower than 70.0 MPa.
 16. Coatingcomposition according to claim 2, wherein the equilibrium modulus ofsaid composition, when cured, is lower than about 1.5 MPa.
 17. Coatingcomposition according to claim 2, wherein the equilibrium modulus ofsaid composition, when cured, is lower than about 1.4 MPa.
 18. Coatingcomposition according to claim 2, wherein the equilibrium modulus ofsaid composition, when cured, is lower than about 1.3 MPa.
 19. A coatedoptical fiber comprising: an optical fiber; and a cured primary coatingon the optical fiber; wherein the primary coating comprises the coatingcomposition according to claim
 1. 20. A coated optical fiber accordingto claim 19, wherein the optical fiber is a single mode optical fiber.21. A coated optical fiber comprising: an optical fiber; a cured primarycoating on the optical fiber; and a cured secondary coating on the curedprimary coating; wherein the primary coating comprises the coatingcomposition according to claim
 1. 22. A coated optical fiber accordingto claim 21, wherein the optical fiber is a single mode optical fiber.